20 GHz Clock Frequency ROM-Less Direct Digital Synthesizer Comprising Unique Phase Control Unit in 0.25 μm SiGe Technology

This paper presents work on a 20 GHz ROM-less direct digital synthesizer (DDS) with 12-bit phase and 6-bit amplitude resolution in a 0.25μm SiGe technology having ft/fmax 180/220GHz. The DDS is applicable for wireless communications and is capable of generating sinusoidal and complex waveforms up to 10 GHz. The 12-bit reduced size pipeline accumulator has been designed with a novel 6-bit phase control word unit to facilitate digital phase modulation schemes. The DDS has been successfully measured and characterized using a dedicated PCB board. Its operation has been confirmed in terms of frequency settings. The output amplitude from the DDS is lower than expected due to losses in the FR4 PCB. To our knowledge this DDS is the first attempt to employ a digital phase control and is among the fastest DDS in SiGe BiCMOS reported so far. The total power consumption is only 1.54 W. The DDS has an output frequency range from 5 MHz to 10 GHz with worst case spurious free dynamic range (SFDR) of 25 dBc with a maximum clock frequency of 20 GHz. The frequency settling time is below 300 ns.